The invention is a minimally invasive total disc replacement device designed with complimentary easy-to-use insertion and removal instrumentation. The invention offers the surgeon the ability to address Lumbar, thoracic and cervical spinal disc disorders. The invention was designed to allow for either full or partial motion rehabilitation as well as a very quick recovery time. It is secured in place by mechanical means and does not require bone to disc fusion, therefore the success of the operation can be readily determined soon after surgery. It is also self-adjusting and as such, a precise placement and positioning is not required. The design allows for a simple disc removal if needed. Most importantly, the design provides for free movement of the artificial disk within carefully controlled ranges of motion and without extraneous penetration of tissue peripheral to its range of operation.
Spinal Disc Orthopaedics
A natural spinal disc accommodates movement freedom for the body it supports. It allows for bending and flexing, for leaning left and right and for rotating. It also supports varying applied loads associated with human body weight and motions.
When the disc gets damaged, a solution needs to be found that mimics the spine in its natural behavior. It needs to accommodate the movement and the applied loads. In some cases (e.g. trauma) there is a benefit in limiting some of the degrees of freedom. As important to the patient, is the requirement that the solution is done using a minimally invasive procedure for implantation. This allows for a quick patient recovery in a matter of days rather than what existing technologies provide. Lastly, the procedure needs to be reversible to allow for unforeseen circumstances and to further lower any risk. The current technologies and procedures can be divided into three categories: Disc repair, disk fusion and disc replacement.
a. Disc Repair:
The procedure calls for surgically removing the fragments of the ruptured disc or removing the herniated part of the disc, which pushes against the nerves causing paralyzing pain and leaving the so called “healthy” part of the disc in place. Although the procedure has advanced greatly and is being performed by microsurgery, allowing relatively quick recovery time, it still has its old problems such as reoccurring herniated or fragmented disc associated with need for repeated surgery, disc thickness reduction and eventually adjacent vertebrae touching each other leading to undesirable permanent disc fusion. In time, the problem often migrates to adjacent discs.
b. Disc Fusion:
The established solution to disc problem is to fuse the upper and lower vertebrae and in most cases adjacent vertebrae by means of mechanical brackets and screws. Some of the problems associated with this approach are: (i) loss of natural body motion, (ii) surrounding organs such as muscles may be permanently damaged and may cause lasting pain for the patient and (iii) very long recovery time. An alternate solution is to apply a spring in a sleeve connector on the outside of the affected vertebrae to allow for some movement. This solution still suffers from drawbacks similar to regular fusion.
c. Artificial Disc Replacement:
The new trend in technology is to replace the disc with an artificial disc, which has two metallic sides with spikes or keels imbedded in the vertebra. That metallic face of the disc touches the adjacent vertebra and promotes bone growth into the metallic disc hence securing it in place. Some of the problems with this approach are:
An approximately two years wait for the bone to grow into the metallic disc and only at that time the success or failure of the operation can be determined. In the case of a successful operation, the fused disc cannot be easily removed if needed. Operation success rate is only about 80%, which is not an acceptable rate for medical implants. A very precise disc placement during the operation is required. Extensive surgeon training is required, which is a considerable drawback for the medical society.
There have been attempts to mitigate these problems by the design of specialized devices. In U.S. Pat. No. 4,599,086 to Doty, a device is disclosed where fixed pins hold the prosthetic vertabrae in a fixed position relative to the neighboring vertebrae. The pins are shown entering the faces of the vertebrae. This design limits movement and, as contrasted with the invention disclosed here, requires more extensive surgical procedures in order to install it. The invention at hand does not require pins on the faces of the vertebrae and in fact is designed to permit the natural movement of the spine. U.S. Pat. No. 6,190,388 to Michelson discloses prosthetic fusion devices that are inserted between the vertebrae whereby the devices are fixed in position by an attachment member. In that patent, a spinal rod is used to hold the fusion devices in place. In this design, motion is limited by the spinal rod and the intent is to promote fusion of the vertebrae—which permanently fixes their position relative to each other. The invention at hand is designed to maintain the motion of the vertebrae by positioning the prosthetic disk using a movable hinging system where multi-dimensional motion is permitted within fixed limits defined by the dimensions of the components. U.S. Pat. App. No. 10754042, by Ferree discloses an artificial disk whose position is maintained between the vertebrae by two metal annulus components that are attached to the opposing faces of the vertebrae. In addition, a cable device is used to assist in the positioning of the disk. This approach also requires extensive surgical interaction because the opposing faces of the vertebrae must be accessed in order to implant the two metal annulus components. In addition, the position of the disk is only limited by the protrusions from the annulus, thus introducing a design trade-off between the danger of the disk falling out of place against the likelihood that the protrusions will be too long thus impacting the neighboring tissue.
U.S. Pat. No. 6,805,714 to Sutcliffe discloses a spinal implant designed to fix the neighboring vertebrae in cases where one of the vertebrae themselves are damaged. This device is designed to fix the position of the vertebrae and promote spinal fusion. This implant design also requires extensive surgical interaction in order that the plates be installed on the opposing faces of the neighboring vertebrae. U.S. Pat. No. 6,846,326 to Cauthen discloses a prosthesis to replace the disk that is comprised of two plates that attach to the opposing faces of the neighboring vertebrae, respectively, using bone fusion. Where the two plates meet, an articulation means permits rocking and twisting motion. This design requires that the vertebrae be fixed during the period that bone fusion with the plates occurs. In addition, the design involves complete removal of the natural tissue in the area.
In U.S. Pat. Application Publication No. 2003/0204260, to Ferree, the description shows an artificial disk prosthesis that is connected by a rigid link to a mounting plate. This design shows that the link member, when moving back and forth with a piston action, extends out from the plate into the surrounding tissue, causing damage and possibly pain. The link member is shown as an integral part of the disk, that is, the rod is fixedly attached to the disk. The design also shows that angulation motion can occur as a result of the link member hinging within a vertical slot. With this approach, the amount of angulation is difficult to control: the geometry requires both tiny machining tolerances to have a properly limited motion while at the same time the backing plate has to be thick enough that the angular stress of the link does not twist it. Ferree also discloses using a swivel to attach the disk to the link, with a vertical pin permitting angulation in the horizontal plane by both the link member and also by the disk. This approach increases the angular range of motion but with no means to accurately control it. This increases the risk that the artificial disk will fall out of place necessitating further surgery. In addition, this approach fixes the link to the disk along the longitudinal axis of the link, which necessitates the link protruding out of the back of the backing plate when the disk moves toward it.